Wave generating circuits



Oct. 14, 1952 A, M SKELLETT 2,614,222

WAVE GENERATING CIRCUITS Original Filed April 24, 1943 2 SHEETS--SHEET l F/G. F/G. 2 EF our =07 v IM 11 5 7 Qu nn- L OUTPUT FIG. 7

ATTORNEY Get. 114 31952 SKELLETT 2,614,222

WAVE GENERATING CIRCUITS GRID PO TENT/AL I l/A R/A T/ON I axe/0 POTENT/AL 1 VA R/A r/o/v F/G. 9 i

PLA TE PO TENT/AL VAR/A T/ON POTENTIAL VAR/A T/ON OF PLATE OE TUBE 7 0 (FIG. .3)

/A/l/EN7OR A. M. SKELLETT Z M ag aq A TTORNE V Patented Get. 14, 1952 WAVE GENERATING CIRCUITS Albert M. Skellett, Madison, N. .l., assignor-to Bell Telephone Laboratories, Incorporated, New York, N. 1 a corporation of New York Application June 9, 1949, Serial No. 97,954, which is a division'of application Serial No. 484,324, April 24, 1943. Divided and this application July 31, 1951, Serial No. 239,454

2 Claims.

The invention relates to Wave generation and particularly to alternating wave generating circuits of the multivibrator type.

This application is a division of my copending application, Serial No. 97,954, filed June 9, 1949, which in turn is a division of my original application, Serial No. 484,324, filed April 24, 19 13, which issued as Patent No. 2,501,620 on March 21, 1950.

The so-called multivibrator is an arrangement of electric space discharge tubes operating as a distorted wave oscillator to produce a discontinuous wave the frequency of which may be adjusted to have any value within wide limits. In its original form as devised by H. Abraham and E. Bloch and described in an article by them in the Annalen de Physique, volume 12, page 237, 1919, it comprises a two-stage resistance-capacity coupled vacuum tube amplifier in which the anode of each tube is capacitively coupled to the control grid of the other tube. The period of oscillation of such a circuit is primarily determined by the time constant of the combination of coupling condenser and the grid resistor through which it discharges in the course of half a cycle.

An object of the invention is to provide the multivibrator type of operation in wave generating circuit Without the use of capacitance for timing control.

Another object is to generate alternatingcurrent waves of any desired stable frequency at a high voltage.

Another object is to generate alternating pulses of various wave forms.

A more specific object is to generate alternatin pulses closely approximating a sine Wave in form.

These objects are obtained in accordance with the invention by the use of combinations of inductance and resistance to perform the functions of the condenser-resistance arrangements in the usual multivibrator circuit as well as to obtain other useful characteristics not ordinarily obtainable with such a circuit. In one embodiment, the modified multivibrator circuit may comprise a pair of multielectrode electron discharge tubes connected in a symmetrical or unsymmetrical circuit arrangement with hi hly resistive impedance coupling between the plate of one or both tubes and the control grid of the other tube, and an inductance coil in the control grid or plate circuit of one or both tubes, the time constant or" the inductance coils and the resistive impedances with which they work primarily determining the period of oscillation of the multivibrator circuit.

The various objects and featuresof the inven- 2 tion'will be better understood from the fcllowing detailed description when read in conjunction with the accompanying drawing in which:

Figs. 1 to 7 show schematically wave generating circuits embodying. different modifications of the invention; and

Figs. 8 to 10 show'curves illustrating typical Wave forms for the control grid and anode potentials of the tubes in the circuits of Figs. 1 to 3 obtained by actual tests of those circuits.

The symmetrical multivibrator circuit of Fig. 1 includes the two three-electrode amplifying vacuum tubes i and 2. The anode or plate of tube 1 is connected directly to the control grid of tube 2 through the highly resistive impedance (resistor) 3, and the anode or plate of tube 2 is connected directly to the control grid of tube 1 through an equivalent highly resistive impedance (resistor) 4. The cathodes of the tubes i and 2 may be heated to incandescence from any suitable source (not shown) which'may be a direct-current battery. The control grid-cathode circuit of tube 1 comprises the cathode resistor 5 and the inductance coil 6 in series, and the control grid-cathode circuit of the tube 2 comprises the same cathode resistor 5 and another inductance coil i, equivalent to the coil 8, in series. Space current is supplied from the common plate battery 9 in parallel to the plates of tubes I and 2 through the individual equivalent series resistors 9 and i0. respectively.

Let it be assumed that the plate battery 8 and cathode battery sources of the tubes l and 5 have been connected as described so that the circuit is oscillating, and that the discharge has just been transferred from tube to tube 1. The potential of the plate of tube i will then be negative with respect to the supply voltage from battery 8, and the control grid of tube 1 will be at or near zero potential with respect to the cathode of that tube. The control grid potential of the other tube 2 will then be more negative than the cut-off value and the current will. be building up in the inductance 1 in the control gridcathode circuit of the tube 2 thus gradually bringing up the potential of the grid of that tube toward thecut-off value. The potential of the plate of tube 2 will be the same as the supply voltage from battery 8.

As the current in inductance coil 1 increases, the potential across it decreases until the potential of the control grid of tube 2 is brought up to cut-off and current starts to flow through that tube. This will cause the potential of the plate of tube 2 to drop, and this potential drop will be transmitted through the coupling resistor'd 3 to the control grid of tube I. since this happens suddenly, the inductance coil 6 in the control grid-cathode circuit of tube I acts as a very high impedance, much higher than that of the coupling resistor 4, so that most of the voltage drop is impressed on the control grid of tube I. Tube I amplifies this voltage change and its plate potential starts to rise toward that of the supply voltage from battery 8. This rise of plate potential is transferred through the coupling resistor 3 to the control grid of tube 2, so that tubes I and 2 operate as a two-stage amplifier. This accelerated action quickly transfers the discharge to tube I and the cycle repeats. The potentials of the cathodes of tubes I and 2 do not vary appreciably during each cycle.

The upper and lower curves of Fig. 8 respectively show the wave form (variation of amplitude with time) of the grid and plate potentials for one of the tubes of the circuit of Fig. 1. The period of oscillation of the multivibrator circuit is determined approximately by the time constant of the coils 6, I and the resistances 4, 3 they work with. That is, the period where L is the inductance of a coil and R is the resistance of a coupling resistor.

The constant frequency wave generated by the circuit of Fig. 1 may be taken off at any point, for example, through a resistance-capacity coupling from one plate circuit, as shown.

Fig. 2 shows a modified symmetrical multivibrator circuit in accordance with the invention differing essentially from that of Fig. 1 merely in that the inductance coils 6 and 1 are connected in push-pull relation withrespect to each other, respectively, in series with the individual portions of the plate-cathode circuits of tubes I and 2 in place of the resistors 9 and I0, instead of in series with the control grid-cathode circuits of those tubes as in Fig. 1; the equivalent resistors 20 and 2| are connected in the control grid-cathode circuits of tubes I and 2, respectively, in series with the common cathode resistor and the generated wave is taken off through another coil II, symmetrically inductively coupled to the coils 6 and I. As shown by the curves of Fig. 9, in the circuit of Fig. 2 the grid potentials are of square wave form and the plate potentials swing over a rather large voltage range, greater than the supply voltage.

Fig. 3 shows an unsymmetrical multivibrator arrangement in accordance with the invention differing from the circuit of Fig. 2 in the following particulars: The connection between the plate of tube 2 and the grid of tube I through resistor 4 is eliminated, the connection between the plate of tube I and the grid of tube 2 through series resistor 3 being retained. The inductance coil 6 is connected in series with the individual portion of the plate-cathode circuit of tube I as in Fig. 2, but the series coil in the individual portion of the plate-cathode circuit of tube 2 is eliminated. A potentiometer 22 having its resistance element connected between the grid resistor 20 and the positive terminal of battery 8 and its variable arm connected to the grid of tube I, is provided for obtaining in conjunction with battery 8 and cathode resistor 5 a negative grid bias of suitable value in tube I. The circuit is driven by an alternating control wave, for example, a wave comprising a series of sharp pulses recurring at regular intervals so as to provide trigger operation, applied to the control grid-cathode circuit of tube I through the resistance-capacity coupling I2; and the generated wave output of the multivibrator is taken off through a coil I3 inductively coupled to the coil 6.

In the circuit of Fig. 3, the tube 2 is normally conducting and the tube I is normally cut off (non-conducting) by the resultant negative bias applied to its grid from battery 8 through the discharge path of tube 2 with that tube conducting, cathode resistor 5 and potentiometer 22 with proper adjustment of its variable arm. The circuit is triggered off by the application of each control pulse to the control grid-cathode circuit of tube I. The trigger input suddenly raises the potential of the control grid of tube I so that the tube starts to conduct immediately. This action is so sudden that the plate impedance of tube I at this time is very high, and the plate potential starts to drop, causing the potential of the control grid of tube 2 to be pushed down through the coupling resistor 3. This lowers the current flow through tube 2 and hence the current through the common cathode resistor 5 dropping the cathode potential of that tube. This action effectively decreases the grid bias on tube I and hastens the transfer of the discharge from tube 2 to tube I through this regenerative action. In this case, the cathode resistor 5 acts to provide positive feedback.

This action continues at a very fast rate until the discharge has transferred to tube I. The current continues to build up in the coil 6 in the plate-cathode circuit of tube I, raising the plate potential of tube I and thus the potential on the grid of tube 2 through coupling resistor 3. When the potential of the grid of tube 2 is built up through this action to th cut-off value, tube 2 starts to conduct. This increases the current flow through the common cathode resistor 5 increasing the bias on tube I, and through the same action as described above, but in the opposite sense, the discharge quickly transfers to the grid of tube 2 to remain there until the next trigger pulse comes along.

The duration of the pulse thus generated in the coil 6 is determined approximately by the time constant of that coil 6 in series with the tube resistance. When the discharge transfers to tube 2, a very high potential is generated in the coil, as indicated by the curve of Fig. 10. In order to generate a high voltage output wave by this means, the coil '6 may be used as the primary winding of a step-up transformer, and the coil I3 in the output circuit, inductively coupled to coil 6 as the secondary winding of that transformer. By the use of a small double triode for tubes I and 2, sparks of several thousand volts have been obtained with the circuit of Fig. 3. If the bias on the control grid of tube I is increased, this circuit, like those of Figs. 1 and 2, will oscillate by itself when the batteries are connected to the tubes.

The coil 6 in the circuit of Fig. 3 may be the operating winding of an electromagnetic relay, in which case the relay will remain operated for a definite time, no matter what the duration of the trigger pulse, provided, of course, that it is less than the duration of the generated pulse. Thus the circuit may operate as a hold-up circuit. Another use might be that of pulse preshaping in telegraphy, etc.

Fig. 4 shows another unsymmetrical multivibrator circuit embodying the invention, differing essentially from that of Fig. 3 in that the resistor 9 is substituted for the inductance coil =6 in series with the plate-cathode circuit of tube I and an inductance coil 7 is connected in place of the series resistor 2| (shown in Fig. 3) between the control grid of tube 2 and a variable contact on the common cathod resistor 5. This multivibrator circuit is triggered off by pulses applied by the saturable magnetic core input transformer M to the control grid-cathode circuit of tube I, and the generated wave is supplied to an output circuit through coil IS inductively coupled to coil 1.

The operation of the circuit of Fig. 4 is similar to that of the circuit of Fig. 3 described above. The cathode resistor 5 again provides positive feedback, and the duration of the square pulse generated by the multivibrator is determined by the resistances which are effectively in series with the coil 1 when the current is building up through it (and the voltage across it is decreasing). Like the circuits of the other figures, the circuit of Fig. 4 will self-oscillate providing the biases applied to the grids of both tubes are su-filcient to make them both conducting at once under which condition the circuit set-up is unstable and acts like a two-stage amplifier with positive feedback.

The unsymmetrical multivibrator circuit of Fig. 5 difiers essentially from that of Fig. 3 in that the coupling from the plate of tube 1 to the control grid of tube 2 is obtained by a coil I6 coupled to the coil 5 in the plate-cathod circuit of tube I, that is, by a transformer formed by these two coils instead of by the resistor 3. As in the circuit of Fig. 3, the trigger pulse is applied to the control grid-cathode circuit of tube I through a resistance-capacity coupling l2, and the output is taken off from the plate circuit through another resistance-capacity coupling 23 instead of through a coil inductively coupled to the coil 6. The operation of the circuit of Fig. 5 will be obvious from that described for the similar circuits of Figs. 3 and 4.

Figs. 6 and '7 show modifications of the multivibrator circuits of Figs. 1 and 2, respectively, which will be effective to make the outputs of these circuits closely approximately sine waves. Fig. 6 differs from the circuit of Fig. 1 merely in addition of the condensers I! and I8, connected across the resistances 9 and IB, respectively, in series with the plate-cathode circuits of tubes l and 2, and Fig. '7 diifers from Fig. 2 merely in the addition of the condenser l9 connected across the plate circuit inductance coils 6 and 1 in series. The function of the added elements is to smooth out the abrupt transitions from one tube to the other and thus to enable sine wave outputs to be produced. The arrangement of Fig. '7 particularly should produce a very good sine wave.

It is obvious that the principles of the invention are applicable to multivibrator circuits employing any even number of tubes, or electron discharge tubes having more than three electrodes, as well as to multivibrator frequency step-up and step-down circuits, in which case the control oscillation injected into the grid or plate circuit would be of a frequency which is a submultiple or a harmonic of the multivibrator frequency, respectively.

Other modifications of the circuits illustrated and described which are within the spirit and scope of the invention will occur to persons skilled in the art.

What is claimed is:

1. An oscillation generator of the multivibrator type comprising a pair of like electron discharge tubes each having a cathode, an anode and a control grid, control rid-cathode circuits for said tubes including a common cathode resistor and two equivalent resistors one of which is connected in series with said common resistor between the control grid and cathode of each of said tubes, anode-cathode circuits for said tubes connected in push-pull relation with respect to each other, including a common source of space current and two equivalent inductance coils one of which is connected between said source and the anode of each of said tubes, a capacitor shunted across the two inductance coils in series, equivalent resistance connections between the anode of each tube and the control grid of the other tube and an output circuit symmetrically coupled to the two anode-cathode circuits for taking oiT the generated oscillations.

2. A multivibrator comprising two like electron discharge tubes each having a heated cathode, an anode and a control grid, control grid-cathode circuits for said tubes including a common cathode resistor and two equivalent resistors respectively connected in series therewith between the control grid and cathode of each tube, anodecathode circuits for said tubes including a common source of space current and two equivalent inductance coils respectively connected between said source and the anode of each of said tubes, a connection including resistance only connected between the anode of each tube and the control grid of the other tube, said multivibrator being capable of self-oscillation at a frequency determined by the time constants of said inductance coils and the associated resistances, a capacitor connected in shunt with said two inductance coils in series, such as to smooth out the abrupt transitions of discharge from one tube to the other and thus to make the generated oscillations closely approximate sine wave form and an output circuit symmetrically inductively coupled to said two intductance coils for taking off the generated oscilla ions.

ALBERT M. SKELLETT.

No references cited. 

